The three-dimensional structure of sugar-binding proteins, essential components of some bacterial transport systems, is currently being investigated by X-ray crystallographic technique. The structure of one of these, L-arabinose-binding protein has been determined at a nominal resolution of 3.5 A. The single polypeptide binding protein (MW equals 33,000) appears to be ellipsoidal with an axial ratio of about 2:1 and consists of two, more or less globular domains. Five helices of at least 2 turns have been located in the Fourier map. One domain contains an extensive pleated sheet structure. A tentative trace of the entire course of the polypeptide chain has been made. The structure analysis of L-arabinose-binding protein is currently being extended to higher resolution where it is possible to fit the complete amino acid sequence of the protein. BIBLIOGRAPHIC REFERENCES: Quiocho, F.A., Structure and Function of Sugar-Binding Proteins, ICRS Science: Cell and Membrane Biol. 4, ii (1976) (Abstract). Phillips, G.N., Jr., Mahajan, V.K., Siu, A.K.Q., and Quiocho, F.A., Structure of L-Arabinose-Binding Protein from Escherichia coli at 5 A Resolution and Preliminary Results at 3.5 A, Proc. Natl. Acad. Sci. USA. 73,2186 (1976).